Very High Energy Observations of Gamma-Ray Burst Locations with the Whipple Telescope D

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Very high energy observations of gamma-ray burst locations with the whipple telescope D. Horan, R. W. Atkins, H. M. Badran, G. Blaylock, S. M. Bradbury, J. H. Buckley, K. L. Byrum, O. Celik, Y. C. K. Chow, P. Cogan, W. Cui, M. K. Daniel, I. D. Perez, C. Dowdall, A. D. Falcone, D. J. Fegan, S. J. Fegan, J. P. Finley, P. Fortin, L. F. Fortson, G. H. Gillanders, J. Grube, K. J. Gutierrez, J. Hall, D. Hanna, J. Holder, S. B. Hughes, T. B. Humensky, G. E. Kenny, M. Kertzman, D. B. Kieda, J. Kildea, H. Krawczynski, F. Krennrich, M. J. Lang, S. LeBohec, G. Maier, P. Moriarty, T. Nagal, R. A. Ong, J. S. Perkins, D. Petry, J. Quinn, M. Quinn, K. Ragan, P. T. Reynolds, H. J. Rose, M. Schroedter, G. H. Sembroski, D. Steele, S. P. Swordy, J. A. Toner, L. Valcarcel, V. V. Vassiliev, R. G. Wagner, S. P. Wakely, T. C. Weekes, R. J. White, and D. A. Williams

This paper is posted at Purdue e-Pubs. http://docs.lib.purdue.edu/physics articles/357 The Astrophysical Journal, 655:396Y405, 2007 January 20 A # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.

VERY HIGH ENERGY OBSERVATIONS OF GAMMA-RAY BURST LOCATIONS WITH THE WHIPPLE TELESCOPE D. Horan,1 R. W. Atkins,2 H. M. Badran,3 G. Blaylock,4 S. M. Bradbury,5 J. H. Buckley,6 K. L. Byrum,7 O. Celik,8 Y. C. K. Chow,8 P. Cogan,9 W. Cui,10 M. K. Daniel,9 I. de la Calle Perez,11 C. Dowdall,9 A. D. Falcone,12 D. J. Fegan,9 S. J. Fegan,8 J. P. Finley,10 P. Fortin,13 L. F. Fortson,14 G. H. Gillanders,15 J. Grube,5 K. J. Gutierrez,6 J. Hall,2 D. Hanna,16 J. Holder,5 S. B. Hughes,6 T. B. Humensky,17 G. E. Kenny,15 M. Kertzman,18 D. B. Kieda,2 J. Kildea,16 H. Krawczynski,6 F. Krennrich,19 M. J. Lang,15 S. LeBohec,2 G. Maier,5 P. Moriarty,20 T. Nagai,19 R. A. Ong,8 J. S. Perkins,6 D. Petry,21 J. Quinn,9 M. Quinn,20 K. Ragan,16 P. T. Reynolds,22 H. J. Rose,5 M. Schroedter,19 G. H. Sembroski,10 D. Steele,14 S. P. Swordy,17 J. A. Toner,15 L. Valcarcel,16 V. V. Vassiliev,8 R. G. Wagner,7 S. P. Wakely,17 T. C. Weekes,1 R. J. White,5 and D. A. Williams23 Received 2006 April 26; accepted 2006 September 15

ABSTRACT Gamma-ray burst (GRB) observations at very high energies (VHE; E > 100 GeV) can impose tight constraints on some GRB emission models. Many GRB afterglow models predict a VHE component similar to that seen in blazars and plerions, in which the GRB spectral energy distribution has a double-peaked shape extending into the VHE regime. VHE emission coincident with delayed X-ray ﬂare emission has also been predicted. GRB follow-up observations have had high priority in the observing program at the Whipple 10 m gamma-ray telescope, and GRBs will continue to be high-priority targets as the next-generation observatory, VERITAS, comes online. Upper limits on the VHE emission at late times (>4 hr) from seven GRBs observed with the Whipple Telescope are reported here. Subject headinggs: gamma rays: bursts — gamma rays: observations Online material: color ﬁgures

1. INTRODUCTION been found to have fluences ranging from a small fraction of, Since their discovery in 1969 (Klebesadel et al. 1973), gamma- up to a value comparable to, that contained in the prompt GRB emission. This X-ray flare emission has been postulated to arise ray bursts (GRBs) have been well studied at many wavelengths. from a number of different scenarios, including late central en- Although various open questions remain regarding their nature, there is almost universal agreement that the basic mechanism is gine activity, where the GRB progenitor remains active for some time after, or reactivates after, the initial explosion (Kumar & an expanding relativistic fireball, that the radiation is beamed, that Piran 2000; Zhang et al. 2006; Nousek et al. 2006; Perna et al. the prompt emission is due to internal shocks, and that the after- 2006; Proga & Zhang 2006; King et al. 2005), and refreshed glow arises from external shocks. It is likely that Lorentz factors shocks, which occur when slower moving shells ejected by the of a few hundred are involved, with the radiating particles, either central engine in the prompt phase catch up with the afterglow electrons or protons, being accelerated to very high energies. shock at late times (Rees & Me´sza´ros 1998; Sari & Me´sza´ros GRBs are subclassified into two categories, long and short burst, 2000; Granot et al. 2003; Guetta et al. 2007). For short GRBs, based on the timescale over which 90% of the prompt gamma- shock heating of a binary stellar companion has also been pro- rayemissionisdetected. posed (MacFadyen et al. 2005). It is not yet clear whether the Recently, the Swift GRB Explorer (Gehrels et al. 2004) has revealed that many GRBs have associated X-ray flares (Burrows X-ray flares are the result of prolonged central engine activity, refreshed shocks, or some other mechanism (Panateiscu et al. et al. 2005; Falcone et al. 2006). These flares have been detected 2006). A very high energy (VHE; E > 100 GeV) component of between 102 and 105 s after the initial prompt emission and have

1 Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for 13 Department of Physics and Astronomy, Barnard College, Columbia Astrophysics, Amado, AZ; Current address: Argonne National Laboratory, University, NY. Argonne IL. 14 Department of Astronomy, Adler Planetarium and Astronomy Museum, 2 Department of Physics, University of Utah, Salt Lake City, UT. Chicago, IL. 3 Department of Physics, Tanta University, Tanta, Egypt. 15 Department of Physics, National University of Ireland, Galway, Ireland. 4 Department of Physics, University of Massachusetts, Amherst, MA. 16 Department of Physics, McGill University, Montreal, Canada. 5 School of Physics and Astronomy, University of Leeds, Leeds, UK. 17 Enrico Fermi Institute, University of Chicago, Chicago, IL. 6 Department of Physics, Washington University, St. Louis, MO. 18 Department of Physics and Astronomy, DePauw University, Greencastle, IN. 7 Argonne National Laboratory, Argonne, IL. 19 Department of Physics and Astronomy, Iowa State University, Ames, IA. 8 Department of Physics and Astronomy, University of California, Los 20 Department of Physical and Life Sciences, Galway-Mayo Institute of Angeles, CA. Technology, Dublin Road, Galway, Ireland. 9 School of Physics, University College, Belfield, Dublin, Ireland. 21 NASA Goddard Space Flight Center, Greenbelt, MD. 10 Department of Physics, Purdue University, West Lafayette, IN. 22 Department of Applied Physics and Instrumentation, Cork Institute of 11 Department of Physics, University of Oxford, Oxford, UK. Technology, Bishopstown, Cork, Ireland. 12 Department of Astronomy and Astrophysics, Pennsylvania State Uni- 23 Santa Cruz Institute for Particle Physics and Department of Physics, versity, University Park, PA. University of California, Santa Cruz, CA. 396 VHE OBSERVATIONS OF GRB LOCATIONS 397 this X-ray flare emission has also been predicted (Wang et al. a factor of >100 better than GLAST will achieve (3 photons of 2006). 10 GeV in 104 cm2 collection area). This ignores the large solid- Within the standard fireball shock scenario (Rees & Me´sza´ros angle advantage of a space telescope and the possible steepening 1992; Me´sza´ros&Rees1993;Sarietal.1998),manymodels of the observable spectrum because of the inherent emission mech- have been proposed that predict emission at and above GeVen- anism and the effect of intergalactic absorption by pair produc- ergies during both the prompt and afterglow phases of the GRB. tion. There have been many predictions of high-energy GRB These have been summarized by Zhang & Me´sza´ros (2004 and emission in and above the GeV energy range (Me´sza´ros et al. references therein) and include leptonic models in which gamma 1994; Boettcher & Dermer 1998; Pilla & Loeb 1998; Wang et al. raysareproducedbyelectronselfYinverse Compton emission 2001; Zhang & Me´sza´ros 2001; Guetta & Granot 2003b; Dermer from the internal shocks or from the external forward or reverse & Atoyan 2004; Fragile et al. 2004; see also Zhang & Me´sza´ros shocks. Other models predict gamma rays from proton synchro- 2004 and references therein). tron emission or photomeson cascade emission in the external Until AGILE and GLAST are launched, the GRB observa- shock or from a combination of proton synchrotron emission and tions that were made by the Energetic Gamma-Ray Experiment photomeson cascade emission from internal shocks. Telescope (EGRET) on the Compton Gamma-Ray Observatory Although GRB observations are an important component of (CGRO) will remain the most constraining in the energy range the program at many VHE observatories, correlated observations from30MeVto30GeV.AlthoughEGRETwaslimitedbya at these short wavelengths remain sparse, even though tantaliz- small collection area and large dead time for GRB detection, it ing and inherently very important. The sparsity of observations made sufficient detections to indicate that there is a prompt of GRBs at energies above 10 MeV is dictated not by lack of component with a hard spectrum that extends at least to 100 MeV interest in such phenomena or the absence of theoretical pre- energies. The average spectrum of four bursts detected by EGRET dictions that the emission should occur, but by experimental (GRBs 910503, 930131, 940217, and 940301) did not show any difficulties. evidence for a cutoff up to 10 GeV (Dingus 2001). The relative For the observation of photons of energies above 100 GeV,only insensitivity of EGRETwas such that it was not possible to elim- ground-based telescopes are available at present. These ground- inate the possibility that all GRBs had hard components (Dingus based telescopes fall into two broad categories, air shower arrays et al. 1998). EGRET also detected an afterglow component from and atmospheric Cerenkov telescopes (of which the majority are GRB 940217 that extended to 18 GeV for at least 1.5 hr after the imaging atmospheric Cerenkov Telescopes [IACTs]). The air prompt emission, indicating that a high-energy spectral compo- shower arrays, which have wide fields of view, making them nent can extend into the GeV band for a long period of time, at particularly suitable for GRB searches, are relatively insensitive. least for some GRBs (Hurley et al. 1994). The spectral slope of There are several reports from these instruments of possible TeV this component is sufficiently flat that its detection at still higher emission. Padilla et al. (1998) reported possible VHE emission at energies may be possible (Mannheim et al. 1996). Me´sza´ros & E > 16 TeV from GRB 920925c. While finding no individual Rees (1994) attribute this emission to the combination of prompt burst that is statistically significant, the Tibet-AS Collaboration MeV radiation from internal shocks with a more prolonged GeV found an indication of >10 TeV emission in a stacked analysis inverse Compton component from external shocks. It is also pos- of 57 bursts (Amenomori et al. 2001). The Milagro Collabora- tulated that this emission could be the result of inverse Compton tion reported the detection of an excess gamma-ray signal during scattering of X-ray flare photons (Wang et al. 2006). Although the prompt phase of GRB 970417a with the Milagrito detector somewhat extreme parameters must be assumed, synchrotron (Atkins et al. 2000). In all of these cases, however, the statistical self-Compton emission from the reverse shock is cited as the significance of the detection is not high enough to be conclusive. best candidate for this GeVemission by Granot & Guetta (2003), In addition to searching the Milagro data for VHE counterparts for given the spectral slope that was recorded. This requirement of 25 satellite-triggered GRBs (Atkins et al. 2005), the Milagro such extreme parameters naturally explains the lack of GRBs for Collaboration conducted a search for VHE transients of 40 s to which such a high-energy component has been observed. Guetta 3 hr duration in the northern sky (Atkins et al. 2004). No evi- & Granot (2003a) postulate that some GRB explosions occur in- dence for VHE emission was found in either of these searches, side pulsar wind bubbles. In such scenarios, afterglow electrons and upper limits on the VHE emission from GRBs were derived. upscatter pulsar wind bubble photons to higher energies during Atmospheric Cerenkov telescopes, particularly those that utilize the early afterglow, thus producing GeV emission such as that the imaging technique, are inherently more flux-sensitive than air observed in GRB 940217. shower arrays and have better energy resolution, but are limited The GRB observational data are extraordinarily complex, and by their small fields of view (3Y5) and low duty cycle (7%). there is no complete and definitive explanation for the diversity In the Burst and Transient Source Experiment (BATSE; Meegan of properties observed. It is important to establish whether there et al. 1992) era (1991Y2000), attempts at GRB monitoring were is, in general, a VHE component of emission present during either limited by slew times and uncertainty in the GRB source position the prompt or afterglow phase of the GRB. Understanding the (Connaughton et al. 1997). nature of such emission will provide important information about Swift, the first of the next generation of gamma-ray satellites, the physical conditions of the emission region. One definitive ob- which will include AGILE (Astro-rivelatore Gamma a Immagini servation of the prompt or afterglow emission could significantly L’Eggero) and the Gamma-Ray LargeAreaSpaceTelescope influence our understanding of the processes at work in GRB (GLAST ), is providing arcminute localizations so that IACTs emission and its aftermath. arenolongerrequiredtoscanalargeGRBerrorboxinorderto In this paper, the GRBs observed with the Whipple 10 m achieve full coverage of the possible emission region. The gamma-ray telescope in response to HighEnergyTransient work in this paper covers the time period prior to the launch of Explorer-2 (HETE-2)andInternational Gamma-Ray Astro- the Swift satellite. physical Laboratory (INTEGRAL) notifications are described. The minimum detectable fluence with an IACT, such as the The search for VHE emission is restricted to times on the order Whipple 10 m telescope, in a 10 s integration is <108 ergs cm2 of hours after the GRB. In x 2, the observing strategy, telescope (5 photons of 300 GeV in a 5 ; 108 cm2 collection area). This is configuration, and data analysis methods used in this paper are 398 HORAN ET AL. Vol. 655

TABLE 1 Properties of the Gamma-Ray Bursts Described in This Work

a b Fluence t > T90 Energy Band GRB Discovery Satellite Trigger Number z (ergs cm2) (s) (keV) (1) (2) (3) (4) (5) (6) (7)

021112...... HETE-2 2448 ... 2.1 ; 107 6.39 30Y400 021204...... HETE-2 2486 ...... 021211...... HETE-2 2493 1.006c 2.4 ; 106 2.80 30Y400 030329...... HETE-2 2652 0.17d 1.1 ; 104 22.76 30Y400 030501...... INTEGRAL 596 ... 1.1 ; 106 75e 25Y100 031026...... HETE-2 2882 6.67f 2.8 ; 106 31.97 30Y400 040422...... INTEGRAL 1758 ... g 8h ...

a The fluence, where available, is quoted for the energy range given in col. (7) over the duration listed in col. (6). For most HETE-2 bursts, this was found at http://space.mit.edu/HETE/Bursts/Data. b Except when a footnote is referenced, the durations in this column are t > T90, the time interval during which 90% of the GRB photons were detected in the 30Y400 keV energy band. c Vreeswijk et al. (2003). d Greiner et al. (2003). e The fluence and duration given in the table are from burst observations with the Ulysses satellite and the SPI-ACS instrument on INTEGRAL. The event was quite weak, so there is a factor of 2 uncertainty in the numbers quoted (Hurley et al. 2003). Observations with the IBIS/ISGRI instrument on INTEGRAL alone gave a duration of 40 s for the burst (Mereghetti et al. 2003a). f This redshift was determined using the redshift estimator described in Pe´langeon et al. (2006). g The fluence was not quoted for this burst over its 8 s duration. It had a fluence of 2:5 ; 107 ergs cm2 when integrated over 1 s Mereghetti et al. (2003b). h It was not stated by Mereghetti et al. (2003b) whether or not this duration is t > T90. described. The properties of the GRBs observed and their ob- scope to a Crab-like spectrum during the observations reported servation with the Whipple Telescope are described in x 3. Fi- here was approximately 400 GeV.This is the energy at which the nally, in x 4, the results are summarized and their implications telescope is most efficient at detecting gamma rays and is subject discussed in the context of some theoretical models that predict to a 20% uncertainty. VHE emission from GRBs. The sensitivity of future instruments 2.2. Obser in Strate y such as the Very Energetic Radiation Imaging Telescope Array v g g System (VERITAS) to GRBs is also discussed. Burst notifications at the Whipple Telescope for the observations described here were received via email from the Global Co- 24 2. THE GAMMA-RAY BURST OBSERVATIONS ordinates Network. When a notification email arrived, the GRB location and time were extracted and sent to the telescope tracking 2.1. Telescope Configuration control computer. An audible alarm sounded to alert the observer The observations presented here were made with the 10 m of the arrival of a burst notification. If at sufficient elevation, the gamma-ray telescope at the Fred Lawrence Whipple Observa- observer approved the observations and the telescope was com- tory. Constructed in 1968, the telescope has been operated as manded to slew immediately to the location of the GRB. The an IACT since 1982 (Kildea et al. 2007). In September 2005, the Whipple Telescope slews at a speed of 1 s1 and therefore can observing program at the 10 m telescope was redefined, and the reach any part of the visible sky within 3 minutes. instrument was dedicated solely to monitoring of TeV blazars Seven different GRB locations were observed with the Whip- and searching for VHE emission from GRBs. Located on Mount ple 10 m telescope between 2002 November and 2004 April. Hopkins, approximately 40 km south of Tucson in southern These observations are summarized in Table 1. At the time these Arizona at an altitude of 2300 m, the telescope consists of 248 data were taken, the point-spread function of the Whipple Tele- hexagonal mirror facets mounted on a 10 m spherical dish with scope was approximately 0.1, which corresponds to the field of an imaging camera at its focus. The front-aluminized mirrors view of one PMT. The positional offsets for the GRB observa- are mounted using the Davies-Cotton design (Davies & Cotton tions (see Table 2) were all less than this, so a conventional point- 1957). source analysis was performed. The imaging camera consists of 379 photomultiplier tubes 2.3. Data Analysis (PMTs) arranged in a hexagonal pattern. A plate of light-collecting cones is mounted in front of the PMTs to increase their light- Thedatawereanalyzedusingtheimagingtechniqueand collecting efficiency. A pattern-sensitive trigger (Bradbury & analysis procedures developed and pioneered by the Whipple Rose 2002) generates a trigger whenever three adjacent PMTs Collaboration (Reynolds et al. 1993). In this method, each im- register a signal above a preset level in the constant-fraction age is first cleaned to exclude the signals from any pixels that discriminators. The PMT signals for each triggering event are are most likely the result of noise. The cleaned images are then read out and digitized using charge-integrating analog-to-digital characterized by calculating and storing the first, second, and converters. In this way, a map of the amount of charge in each third moments of the light distribution in each image. The pa- PMT across the camera is recorded for each event and stored rameters and this procedure are described elsewhere (Reynolds for offline analysis. The telescope triggers at a rate of 25 Hz et al. 1993). Since gamma-ray images are known to be compact (including background cosmic-ray triggers) when pointing at high (>50 ) elevation. Although sensitive in the energy range 24 See the homepage of the Gamma-Ray Bursts Coordinates Network, from 200 GeV to 10 TeV, the peak response energy of the tele- http://gcn.gsfc.nasa.gov. No. 1, 2007 VHE OBSERVATIONS OF GRB LOCATIONS 399

TABLE 2 of false triggers or to have large positional errors, the data from VHE GRB Observations observations of seven GRB locations remained. These GRBs

a b c d took place between UT dates 2002 November 12 and 2004 April TGRBTOBS Exposure Position Offset TGRBTUL Flux 22; two have redshifts derived from spectral measurements, one GRB (hr) (minute) (deg) (hr) (crab) has an estimated redshift, and four lie at unknown distances. Five 021112...... 4.24 110.56 0.013 5.1 <0.200 of the sets of GRB follow-up observations were carried out in 28.63 55.28 0.013 29.0 <0.303 response to triggers from the High-Energy Transient Explorer 2 021204...... 16.91 55.34 0.009 17.4 <0.331 (HETE-2; Lamb et al. 2000), while two sets of observations were 021211...... 20.69 82.79 0.058 21.9 <0.325 triggered by INTEGRAL (Winkler et al. 1999). In the remainder 030329...... 64.55 65.21 0.060 66.2 <0.360 of this section, the properties of each of the GRBs observed and 112.58 83.17 0.022 113.8 <0.279 the results of these observations are presented. A summary of the 136.23 37.55 0.022 137.0 <0.323 GRB properties is given in Table 1, while the observations taken < 162.14 27.74 0.022 162.4 0.519 at the Whipple Observatory are summarized in Table 2. 186.16 27.73 0.022 186.4 <0.399 030501...... 6.58 83.10 0.001 7.3 <0.265 3.1. GRB 021112 031026...... 3.68 82.70 0.007 4.9 <0.406 040422...... 3.99 27.63 0.062 4.2 <0.620 This was a long GRB with a duration of >5 s and a peak flux of >3 ; 108 ergs cm2 s1 in the 8Y40 keV band (Fenimore a The time in hours between the start of the GRB and the beginning of ob- etal.2002).Inthe30Y400 keV energy band, the burst had a servations with the Whipple 10 m telescope. b The angular separation between the position at which these data were peak energy of 57.15 keV, a duration of 6.39 s, and a fluence 7 2 25 taken and the refined location of the GRB. of 2:1 ; 10 ergs cm . The triggering instrument was the c The length of time after the GRB for which the upper limits ( ULs) are French Gamma Telescope (FREGATE) instrument on HETE-2. quoted. Since all data are combined to compute the upper limit, the mean time The Milagro data taken during the time of this burst were searched of the observations is quoted as the time to which the upper limit pertains. for GeV/TeV gamma-ray emission. No evidence for prompt emis- d This is the flux upper limit in units of equivalent crab flux above the peak response energy of 400 GeV. Above this energy, the integrated crab flux is sion was found, and a preliminary analysis, assuming a differ- (8:412 1:840) ; 1011 cm2 s1. ential photon spectral index of 2.4, gave an upper limit on the fluence at the 99.9% confidence level (c.l.) of JðÞ0:2Y20 TeV < 2:6Y106 ergs cm2 over a 5 s interval (McEnery 2002a). Op- and elliptical, while those generated by cosmic-ray showers tend tical observations with the 0.6 m Red Buttes Observatory Tele- to be broader, with more fluctuations, cuts can be derived on the scope beginning 1.8 hr after the burst did not show any evidence above parameters that reject approximately 99.7% of the back- for an optical counterpart and placed a limiting magnitude of ground images while retaining over 50% of those generated by RC ¼ 21:8(3) on the optical emission; at the time, this was gamma-ray showers. These cuts are optimized using data taken the deepest nondetection of an optical afterglow within 2.6 hr on the Crab Nebula, which is used as the standard candle in the of a GRB (Schaefer et al. 2002). TeV sky. Two sets of observations on the location of GRB 021112 Two different modes of observation are employed at the Whip- were made with the Whipple 10 m telescope. The first obser- ple Telescope, on-off and tracking (Catanese et al. 1998). The vations commenced 4.2 hr after the GRB occurred and lasted choice of mode depends on the nature of the target. The GRB for 110.6 minutes. Observations were also taken for 55.3 minutes data presented here were all taken in tracking mode. Unlike data on the following night, 28.6 hr after the GRB occurred. Upper taken in on-off mode, scans taken in tracking mode do not limits (99.7% c.l.) of 0.20 crab26 and 0.30 crab (E > 400 GeV), have independent control data, which can be used to establish respectively, were derived for these observations, assuming a the background level of gamma-rayYlike events during the scan. Crab-like spectrum (spectral index of 2.49). These control data are essential in order to estimate the number of events passing all cuts that would have been detected during 3.2. GRB 021204 the scan in the absence of the candidate gamma-ray source. In Little information is available in the literature on this HETE-2 order to perform this estimate, a tracking ratio is calculated by burst. The GRB location was observed with a number of opti- analyzing dark-field data (Horan et al. 2002). These consist of cal telescopes (the RIKEN 0.2 m [Torii et al. 2002], the 32 inch on-off off-source data taken in the mode and of observations of [81 cm] Tenagra I [Nysewander et al. 2002], and the 1.05 m objects found not to be sources of gamma rays. A large data- SchmidtatKisoObservatory[Ishiguro et al. 2002]), but no op- base of these scans is analyzed, and in this way the background tical transient was found to a limiting magnitude of R ¼ 16:5, level of events passing all gamma-ray selection criteria can be 2.1 hr after the burst (Torii et al. 2002), and to R ¼ 18:8, 6.2 hr characterized as a function of zenith angle. Since the GRB data after the burst (Ishiguro et al. 2002). described in this paper were taken at elevations between 50 Whipple observations of this burst location commenced 16.9 hr and 80 , a large sample of dark-field data (233 hr) spanning a after the GRB occurred and lasted for 55.3 minutes. An upper similar zenith angle range was analyzed so that the background limit (99.7% c.l.) of 0.33 crab was derived for the VHE emission during the gamma-ray burst data runs could be estimated. above 400 GeV during these observations. 3. THE GAMMA-RAY BURSTS 3.3. GRB 021211 This paper concentrates on the GRB observations made in This long, bright burst was detected by all three instruments response to HETE-2 and INTEGRAL triggers with the Whipple on HETE-2. It had a duration >5.7 s in the 8Y40 keV band, with 10 m gamma-ray telescope; observations made in response to afluenceof106 ergs cm2 during that interval (Crew et al. Swift triggers are the subject of a separate paper (C. Dowdall et al. 2007, in preparation). When the GRB data were filtered to 25 See HETE-2 pages at MIT, http://space.mit.edu /HETE /Bursts/ Data. remove observations made at large zenith angles, during inferior 26 Since the crab is the standard candle in the VHE regime, it is customary to weather conditions, and of positions later reported to be the result quote upper limits as a fraction of the crab flux at the same energy. 400 HORAN ET AL. Vol. 655

2002). The peak flux was >8 ; 107 ergs cm2 s1 (i.e., >20 crab flux) in 5 ms (Crew et al. 2002). This burst had a peak energy of 45.56 keV, a duration of 2.80 s, and a fluence of 2:4 ; 106 ergs cm2 in the 30Y400 keVenergy band.27 Fox et al. (2003) reported the early optical, near-infrared, and radio observations of this burst. They identified a break in the optical light curve of the burst at t ¼ 0:1Y0.2 hr, which was interpreted as the signature of a reverse shock. The light curve comprised two distinct phases. The initial steeply declining flash was followed by emission declining as a typical afterglow with a power-law index close to 1. Katzman Automatic Imaging Telescope (KAIT) observations of the afterglow also detected the steeply declining light curve and evidence for an early break (Li et al. 2003). The optical transient was detected at many observatories (Park et al. 2002; Li et al. 2002; Lamb et al. 2002; Bersier et al. 2002; McLeod et al. 2002). The optical transient faded from an R-band magnitude of 18.3, 20.7 minutes after the burst, to an R-band magnitude of 21.1, 5.7 hr after the burst (Fox et al. 2003). Vreeswijk et al. (2003) derived a redshift of 1.006 for this burst, based on spectroscopic observations carried out with the European Southern Observatory’s Very Large Telescope (VLT) at Paranal, Chile. Milagro searched for emission at GeV/TeV energies over the burst duration reported by the HETE-2 wide-field X-ray mon- itor. They did not find any evidence for prompt emission, and a preliminary analysis, assuming a differential photon spectral index of 2.4, gave an upper limit on the fluence at the 99.9% confidence level of J(0:2Y20 TeV) < 3:8 ; 106 ergs cm2 over a 6 s interval (McEnery 2002b). Whipple observations on this GRB location were initiated 20.7 hr after the GRB and lasted for 82.8 minutes. An upper limit (99.7% c.l.) on the VHE emission of 0.33 crab (E > 400 GeV) was derived from these observations. 3.4. GRB 030329 This GRB is one of the brightest bursts on record. It triggered the FREGATE instrument on HETE-2 in the 6Y120 keVenergy band. It had a duration 22.76 s, a fluence of 1:1 ; 104 ergs cm2, and a peak energy of 67.86 keV in the 30Y400 keV band.28 The peak flux over 1.2 s was 7 ; 106 ergs cm2 s1,whichis Fig. 1.—Top: Flux upper limits above 400 GeV (99.7% c.l.) on the VHE emission from GRB 030329. The time periods during which the four bumps in >100 times the crab flux in that energy band (Vanderspek et al. the light curve occur (Granot et al. 2003) are shown as shaded rectangles. Bottom: 2003). Optical light curve of GRB 030329 taken from Lipkin et al. (2004). The time The optical transient was identified by Peterson & Price (2003). since the GRB is shown with the same scale on the x-axis of both plots. [See the Due to its slow decay (Uemura 2003) and brightness (R 13), electronic edition of the Journal for a color version of this figure.] extensive photometric observations were possible, making this one of the best-observed GRB afterglows to date. Early obser- 2003). The afterglow was also bright at submillimeter (Hoge et al. vations with the VLT (Greiner et al. 2003) revealed evidence for 2003) and near-infrared wavelengths (Lamb et al. 2003). The narrow emission lines from the host galaxy, indicating that this X-ray afterglow was detected by the Rossi X-Ray Timing Ex- GRB occurred at a low redshift of z ¼ 0:1687. Observations of plorer (RXTE) during a 27 minute observation that began 4 hr the afterglow continued for many nights, as it remained bright 51 minutes after the burst (Marshall & Swank 2003). The flux with a slow but uneven rate of decline and exhibited some epi- was 1:4 ; 1010 ergs cm2 s1 in the 2Y10 keV band (0.007% sodes of increasing brightness. These observations are well doc- of the crab). umented in the GCN archives. Spectral measurements made on Whipple observations of the location of GRB 030329 com- 2003 April 6 by Stanek et al. (2003a) showed the development of menced 64.6 hr after the prompt emission. In total, 241.4 minutes broad peaks in flux, characteristic of a supernova. Over the next of observation were taken spanning five nights. The upper limits few nights, the afterglow emission faded, and the features of the (99.7% c.l.) from each night of observation are listed in Table 2 supernova became more prominent (Stanek et al. 2003b). These and are displayed on the same temporal scale as the optical light observations provided the first direct spectroscopic evidence that curve of the GRB afterglow in Figure 1. When these data were at least a subset of GRBs is associated with supernovae. combined, an upper limit (99.7% c.l.) for the VHE emission The afterglow was detected at many other wavelengths. Radio above400GeVof0.17crabwasderived. observations with the VLA detected a 3.5 mJy source at 8.46 GHz. 3.5. GRB 030501 This is the brightest radio afterglow detected to date (Berger et al. This burst was initially detected by the imager on board the 27 See HETE-2 pages at MIT, http://space.mit.edu /HETE/Bursts/ Data. INTEGRAL satellite (IBIS/ISGRI) and was found to have a du- 28 See HETE-2 pages at MIT, http://space.mit.edu /HETE /Bursts/ Data. ration of 40 s (Mereghetti et al. 2003a). The burst was also No. 1, 2007 VHE OBSERVATIONS OF GRB LOCATIONS 401 detected by the Ulysses spacecraft and the spectrometer instru- et al. 2004; Rykoff 2004; Huang et al. 2004; Piccioni et al. ment (SPI-ACS) on INTEGRAL (Hurley et al. 2003). Triangu- 2004; Rumyantsev & Pozanenko 2004; Qiu & Hu 2004). The lation between these two detections allowed a position annulus ROTSE-IIIb Telescope at McDonald Observatory began tak- to be computed for this GRB. As observed by Ulysses, it had a du- ing unfiltered optical data 22.1 s after the GRB. Using the first ration of 75 s and had a 25Y100 keV fluence of approximately 110 s of data, a limiting magnitude of 17.5wasplacedonthe 1:1 ; 106 ergs cm2,withapeakfluxof4:9 ; 107 ergs cm2 s1 R-band emission from the GRB at this time (Rykoff 2004). over 0.25 s. Follow-up optical observations with several tele- Whipple observations of this burst commenced 4.0 hr after scopes did not find evidence for an optical transient (Ofek et al. the prompt emission and continued for 27.6 minutes. An upper 2003; Rumyantsev et al. 2003; Boer & Klotz 2003) to a lim- limit (99.7% c.l.) on the VHE emission (E > 400 GeV) of 0.62 iting magnitude of R ¼ 18:0, 0.3Y17 minutes after the burst crab was derived. (Boer & Klotz 2003), and to a limiting magnitude of R ¼ 20:0, 4. RESULTS AND DISCUSSION 16.5 hr after the burst (Ofek et al. 2003). Whipple observations of this burst location commenced 6.6 hr Upper limits on the VHE emission from the locations of seven after its occurrence and continued for 83.1 minutes. An upper GRBs have been derived over different timescales. For each limit (99.7% c.l.) on the VHE emission (E > 400 GeV) during GRB, a number (1Y10) of follow-up 28 minute duration obser- these observations of 0.27 crab was derived. vations were taken with the Whipple 10 m telescope. These GRB data were grouped by UT day and were combined to give 3.6. GRB 031026 one upper limit for each day of observation. The limits range from 20% to 62% of the crab flux above 400 GeV and are pre- This burst was located by the FREGATEinstrument on HETE-2. sented in Table 2. In addition to calculating upper limits on the ; 6 2 It had a duration of 114.2 s, with a fluence of 2:3 10 ergs cm GRB emission for each day, upper limits were calculated for in the 25Y100 keV energy band (Ricker et al. 2003), while in Y each of the 28 minute scans. These are plotted for each of the the 30 400 keVenergy band it had a duration of 31.97 s and a GRBs in Figure 2. ; 6 2 29 fluence of 2:8 10 ergs cm . Follow-up optical observa- The usefulness of the upper limits presented here is limited tions were carried out with a number of instruments, including by the fact that five of the GRBs occurred at unmeasured red- the 1.05 m Schmidt telescope at the Kiso Observatory (Budi shifts, thus making it impossible to infer the effects of the in- et al. 2003), the 32 inch Tenagra II Telescope (Nysewander frared background light on those observations. In addition to et al. 2003a), and the 1.0 m telescope at the Lulin Observatory this, the earliest observation was not made at Whipple until (Huang et al. 2003), but no optical transient was found to a Y 3.68 hr after the prompt GRB emission. Although the Whipple limiting magnitude of R ¼ 20:9for observations taken 6 12 hr 10 m telescope is capable of beginning GRB observations less after the burst (Huang et al. 2003) and to IC ¼ 20:4forobser- than 2 minutes after receiving notification, a number of factors, vations taken 3.9 and 25.7 hr after the burst (Nysewander et al. including notifications arriving during daylight and delays in 2003b). The 30 m IRAM telescope was used to search the field the distribution of the GRB locations, delayed the commence- around the GRB location, but did not detect any source with a ment of the GRB observations presented here. Although data 250 GHz flux density >16 mJy (Bertoldi et al. 2003). A spec- taking for GRB 031026 began 3.7 minutes after the GRB no- tral analysis of the prompt X-ray and gamma-ray emission from tification was received, this notification was not distributed by this burst revealed it to have a very hard spectrum, which is un- the GCN until 3.3 hr after the GRB had occurred. Thus, the ob- usual for such a long and relatively faint burst (Atteia et al. servations presented here cannot be used to place constraints 2003a). It was noted that the count ratio of >1.8 between the Y Y on the VHE component of the initial prompt GRB emission and 7 30 keV and 7 80 keV FREGATE energy bands was one pertain only to the afterglow emission and delayed prompt emis- of the most extreme measured (Ricker et al. 2003). A ‘‘new sion from GRBs. pseudoredshift’’ of 6:67 2:9 was computed for this burst One of the main obstacles for VHE observations of GRBs is using the prescription of Pe´langeon et al. (2006). the distance scale. Pair production interactions of gamma rays Whipple observations of this burst location were initiated with the infrared photons of the extragalactic background light 3.7 minutes after receiving the GRB notification. The burst no- attenuate the gamma-ray signal, thus limiting the distance over tification, however, was not received until more than 3 hr after which VHE gamma rays can propagate. Recently, however, the prompt GRB emission. Although Whipple observations the HESS telescopes have detected the blazar PG 1553+113 commenced 3.3 hr after the prompt emission, the first data run (Aharonian et al. 2006). The redshift of this object is not known, is not included here due to inferior weather conditions. The data but there are strong indications that it lies at z > 0:25, possibly presented here commenced 3.7 hr after the GRB and continued as far away as z ¼ 0:74. This could represent a large increase in for 82.7 minutes. An upper limit (99.7% c.l.) for the VHE emis- distance to the most distant detected TeV source, revealing more sion (E > 400 GeV) of 0.41 crab was derived. of the universe to be visible to TeV astronomers than was pre- viously thought. Although GRBs lie at cosmological distances, 3.7. GRB 040422 many have been detected at redshifts accessible to VHE ob- This burst was detected by the imaging instrument (IBIS/ servers. Of the GRBs studied here, only two had spectroscopic ISGRI) on the INTEGRAL satellite in the 15Y200 keV en- redshifts measured, while the redshift of one was estimated by ergy band. It had a duration of 8 s, a peak flux between 20 and Pe´langeonetal.(2006)usinganimprovedversionofthered- 200 keV of 2.7 photons cm2 s1, and a fluence (1 s integra- shift estimator of Atteia et al. (2003b). Since all of the GRBs tion time) of 2:5 ; 107 ergs cm2 (Mereghetti et al. 2003b). discussed here were long bursts, it is likely that their redshifts Follow-up observations were carried out by many groups, but are of order 1. Due to the unknown redshifts of most of the no optical transient was detected (Malesani et al. 2004; Maeno bursts and the uncertainty in the density of the extragalactic background light, the effects of the absorption of VHE gamma rays by the infrared background light have not been included 29 See HETE-2 pages at MIT, http://space.mit.edu /HETE /Bursts/ Data. here. Fig. 2.—For each GRB location observed, flux upper limits in units of 1011 ergs cm2 s1, calculated for each28 minute scantaken, plotted here as a function of the time since the GRB prompt emission for each GRB. Only one 28 minute observation was made on GRB 040422, so the plot for this GRB is not shown. [See the electronic edition of the Journal for a color version of this figure.] VHE OBSERVATIONS OF GRB LOCATIONS 403

predictions, the sensitivity is close to that required to detect the emission predicted. A recent analysis of archival data from the EGRET calorim- eter has found a multi-MeV spectral component in the prompt phase of GRB 941017, which is distinct from the lower energy component (Gonza´lez et al. 2003). This high-energy component appeared between 10 and 20 s after the start of the GRB and had a roughly constant flux, with a relatively hard spectral slope for 200 s. This observation is difficult to explain using the standard synchrotron model, thus indicating the existence of new phenomena. Granot & Guetta (2003) investigated possible scenarios for this high-energy spectral component and found that most models fail. They concluded that the best candidate for the emission mechanism is synchrotron self-Compton emission from the reverse shock and predicted that a bright optical transient, similar to that observed in GRB 990123, should accompany this high-energy component. Pe’er & Waxman (2004) explain this high-energy tail as emission from the forward-shock Fig. 3.—Flux upper limits above 400 GeV for all of the GRBs observed (blue electrons in the early afterglow phase. These electrons inverse triangles). The limits are plotted as a function of time since the GRB prompt Compton scatter the optical photons that are emitted by the re- emission. The approximate flux level at 400 GeV predicted by Pe’er & Waxman verse shock electrons resulting in powerful VHE emission for (2004) is indicated by the red solid lines, along with the time interval during which it is predicted to occur. Magenta squares show the emission at 400 GeV predicted 100 to 200 s after the burst, as indicated by the lines on Figure 3. by Zhang & Me´sza´ros (2001) at various times after the GRB prompt emission; the Although the observations presented here did not commence prediction of Guetta & Granot (2003a) for VHE emission at 250 GeV 5 ; 103 s early enough after the prompt GRB emission to constrain such after the burst from the combination of external Compton and synchrotron self- models, the sensitivity of the Whipple Telescope is such that Compton emission is shown by the black star. [See the electronic edition of the Journal for a color version of this figure.] the VHE emission predicted by these models would be easily detectable for low-redshift bursts. The prediction of Guetta & Granot (2003a) for VHE emis- Granot et al. (2003) analyzed the late-time light curve of sion 5 ; 103 s after the burst from the combination of exter- GRB 030329 and find that the large variability observed at sev- nal Compton emission (the relativistic electrons behind the eral times (t ¼ 1:3Y1.7 days, 2.4Y2.8 days, 3.1Y3.5 days, afterglow shock upscatter the plerion radiation) and synchro- and 4.9Y5.7 days) after the burst is most likely the result of tron self-Compton emission (the electrons accelerated in the refreshed shocks. These time intervals have been highlighted afterglow emit synchrotron emission and then upscatter this in the top panel of Figure 1, and it can be seen that some of the emission to the VHE regime) is indicated by a star on Figure 3. observations taken at Whipple occurred during these times, thus Theemissionispredictedtohaveacutoffat250 GeV due imposing upper limits on the VHE emission during these re- to pair production of the high-energy photons with the radia- freshed shocks. Since GRB 030329 occurred at a low redshift tion field of the pulsar wind bubble. For afterglows with an (z ¼ 0:1685), it is possible that the effects of infrared absorption external density similar to that of the interstellar medium, pho- on any VHE emission component may not have been significant tons of up to 1 TeV are possible. It can be seen that, although enough to absorb all VHE photons over the energy range to which the upper limits presented here are below the predicted flux from the Whipple Telescope is sensitive. Guetta & Granot (2003a), the observations at Whipple took Figure 3 shows these scan-by-scan upper limits as a function place after this emission was predicted to have occurred. Had of time since the prompt GRB emission. Also plotted are the data taking at Whipple commenced earlier, the emission pre- predicted fluxes at various times after the GRB by Zhang & dicted by these authors should have been detectable for nearby Me´sza´ros (2001) and Pe’er & Waxman (2004) at 400 GeV, and GRBs. by Guetta & Granot (2003a) at 250 GeV. Although the peak Razzaque et al. (2004) investigated the interactions of GeV response energy of the Whipple Telescope at the time of these and higher energy photons in GRB fireballs and their surround- observations was 400 GeV, it still had sensitivity, albeit some- ings for the prompt phase of the GRB. They predict that high- what reduced, at 250 GeV. energy photons escaping from the fireball will interact with Razzaque et al. (2004) predict a delayed GeV component infrared and microwave background photons to produce delayed in the GRB afterglow phase from the inverse Compton up- secondary photons in the GeVYTeV range. Although obser- scattering on external shock electrons. The duration of such vations of the prompt phase of GRBs are difficult with IACTs, a component is predicted to be up to a few hours, softening since they are pointed instruments with small fields of view, with time. Zhang & Me´sza´ros (2001) investigated the differ- which must therefore be slewed to respond to a burst notifi- ent radiation mechanisms in GRB afterglows and identified cation, observations in time to detect the delayed emission are parameter space regimes in which different spectral compo- possible. nents dominate. They found that the inverse Compton GeV There are many emission models that predict significant photon component is likely to be significantly more important VHE emission during the afterglow phase of a GRB, either than a possible proton synchrotron or electron synchrotron com- related to the afterglow emission itself or as a VHE component ponent at these high energies. The predictions of Zhang & of the X-ray flares that have been observed in many Swift bursts. Me´sza´ros (2001) for VHE emission at different times after a O’Brien et al. (2006) analyzed 40 Swift bursts that had narrow- typical regime II burst are shown by squares on Figure 3. Al- field instrument data within 10 minutes of the trigger and found though the observations presented here do not constrain these that 50% had late (t > T90) X-ray flares. If the bulk of the 404 HORAN ET AL. Vol. 655 radiation comes via synchrotron radiation, as is usually sup- posed, then, by analogy with other systems with similar properties (supernova remnants and active galactic nuclei jets), it is natural to suppose that there must also be an inverse Compton component by which photons are boosted into the GeVYTeV energy range. This process is described by Pilla & Loeb (1998), who discuss the relationship between the energy at which the high-energy cutoff occurs, the bulk Lorentz factor, and the size of the emission region. A high-energy emission component due to inverse Compton emission has also been considered in detail for GRB afterglows by Sari & Esin (2001); the predicted flux at GeVYTeV energies is comparable to that near the peak of the radiation in the afterglow synchrotron spectrum. Only direct observations can confirm whether this is so. Guetta & Granot (2003b) predict that the 300 GeV photons from the prompt GRB phase will interact with background IR photons, making delayed high-energy emission undetectable unless the intergalactic magnetic fields are extremely small. The Swift GRB Explorer has shown that 50% of GRBs Fig. 4.—Sensitivity of the VERITAS array for exposures of 50, 5, 0.5, and have one or more X-ray flares. These flares have been detected 0.05 hr (i.e., 3 minutes). [See the electronic edition of the Journal for a color up to 105 s(28 hr) after the prompt emission (Burrows et al. version of this figure.] 2005). Indeed, the delayed gamma-ray component detected in BATSE bursts (Connaughton 2002) may also be associated with getic, Ramirez-Ruiz et al. (2005) argue that the observations of this phenomenon. Recently, Wang et al. (2006) have predicted GRB 031203 can indeed be the result of off-axis viewing of a VHE emission coincident in time with the X-ray flare photons. typical, powerful GRB with a jet. Should future observations In this model, if the X-ray flares are caused by late central engine prove there to be a closer, less powerful population of GRBs, activity, the VHE photons are produced from inverse Compton these would be prime targets for IACTs. scattering of the X-ray flare photons from forward-shock elec- In the past year, the Whipple Observatory 10 m telescope has trons. If the X-ray flares originate in the external shock, VHE been used to carry out follow-up observations on a number of photons can be produced from synchrotron self-Compton emis- GRBs detected by the Swift GRB Explorer. The analysis of these sion of the X-ray flare photons with the electrons that produced observations will be the subject of a separate paper (C. Dowdall them. Should VHE emission be detected from a GRB coinci- et al. 2007, in preparation). dent with X-ray flares, the time profile of the VHE emission The Very Energetic Radiation Imaging Telescope Array Sys- couldbeusedtodistinguishbetween these two origins of the tem (VERITAS) is currently under construction at the Fred X-ray flares. Lawrence Whipple Observatory in southern Arizona. Two of No evidence for delayed VHE gamma-ray emission was seen the four telescopes are fully operational, and it is anticipated from any of the GRB locations observed here, and upper limits that the four-telescope array will be operational by the end of have been placed on the VHE emission at various times after the 2006. GRB observations will receive high priority and, when prompt GRB emission. Although there are no reports of the de- a GRB notification is received, their rapid follow-up will take tection of X-ray flares or delayed X-ray emission from any of precedence over all other observations. The VERITAS tele- these GRBs, it is likely that such emission was present in at least scopes can slew at 1 s1, thus enabling them to reach any part some of them, given the frequency with which it has been de- ofthevisibleskyinlessthan3minutes.Whenanacceptable tected in GRBs observed by Swift. Indeed, the light curve of (i.e., at high enough elevation) GRB notification is received GRB 030329 shows a large variability in amplitude a few days during observing at VERITAS, an alarm sounds to alert the ob- after the burst and, as shown in Figure 1, Whipple observations server that a GRB position has arrived. After receiving autho- were taken during these episodes. Apart from this, a measured rization from the observer, the telescope slews immediately to redshift is available for only one of the other bursts observed the position and data taking begins. Given that the maximum here, and it is possible that the remaining five occurred at dis- time to slew to a GRB is 3 minutes and that Swift notifications tances too large to be detectable in the VHE regime. can arrive within 30 s of the GRB, it is possible that VERITAS Soderberg et al. (2004) reported an unusual GRB (GRB 031203) observations could begin as rapidly as 2Y4 minutes after the GRB, that was much less energetic than average. Its similarity, in depending on its location with respect to the previous VERITAS terms of brightness, to an earlier GRB (GRB 980425) suggests target. that the nearest and most common GRB events have not been As has been shown above, the Whipple 10 m telescope is detected until now because GRB detectors were not sensitive sensitive enough to detect the GRB afterglow emission pre- enough (Sazonov et al. 2004). Most GRBs that have been stud- dicted by many authors. With its improved background rejec- ied until now lie at cosmological distances. They generate a tion and greater energy range, VERITAS will be significantly highly collimated beam of gamma rays, ensuring that they are more sensitive for GRB observations than the Whipple 10 m powerful enough to be detectable at large distances. Both of the telescope. The VERITAS sensitivity for observations of differ- less powerful GRBs detected to date occurred at considerably ent durations is shown in Figure 4. Based on the assumed rate lower redshifts, GRB 980425 at z ¼ 0:0085 and GRB 031203 of Swift detections (100 yr1), the fraction of sky available to at z ¼ 0:1055. Although Soderberg et al. (2004) conclude that VERITAS, the duty cycle at its site, and the Sun-avoiding point- until now GRB detectors have only detected the brightest ing of Swift, which maximizes its overlap with nighttime obser- GRBs and that the nearest and most common GRB events have vations, it is anticipated that 10 Swift GRBs will be observable been missed because they are less highly collimated and ener- each year with VERITAS. No. 1, 2007 VHE OBSERVATIONS OF GRB LOCATIONS 405

The authors would like to thank Emmet Roache, Joe Melnick, PPARC, and Enterprise Ireland. Extensive use was made of the Kevin Harris, Edward Little, and all of the staff at the Whipple GCN web pages (http://gcn.gsfc.nasa.gov). The web pages of Observatory for their support. The authors also thank the anon- Joachim Greiner and Stephen Holland (http://www.mpe.mpg.de/ ymous referee for his/her comments, which were very useful and ~jcg/grbgen.html and http://lheawww.gsfc.nasa.gov/~sholland/ improved the paper. This research was supported in part by the grb/index.html) proved very useful in tracking down references US Department of Energy, the National Science Foundation, and information related to the GRBs discussed in this paper.

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